Osteoarthritis is a progressive disease of joints that is a cause of serious disability in large numbers of people. The disease is defined as a progressive degeneration of the cartilaginous surfaces of joints that leads to stiffness, pain, and loss of mobility. Degeneration of the cartilaginous surface of joints seen in osteoarthritis can have a number of causes. For example, severe trauma to a joint or a bacterial infection in a joint can produce degeneration of the joint that is either immediate or slowly progressive over many years. A number of metabolic disturbances are also know to produce degeneration of joints.
Cartilage and membranes that line joints are complex structures. A major source of the strength of cartilage is the fibrils of type II collagen. The fibrils of type II collagen are stretched into three-dimensional arcades primarily by the presence of another group of macromolecules called proteoglycans. Proteoglycans are highly charged and, therefore, absorb water and salts and thereby extend the arcades of type II collagen fibrils. As a result, a highly resilient structure is formed that can withstand the intermittent pounding and pressures that joints must undergo. In addition to proteoglycans and type II collagen, cartilage is known to contain at least four other kinds of collagens (types VI, IX, X and XI) in lesser amounts than type II collagen. It is very likely that additional collagens will be discovered in cartilage in the future. In addition, it is clear that the matrix of cartilage also contains a number of other proteins that are still poorly characterized and that may contribute to the structure and function of the tissue.
Collagens, proteoglycans and other proteins found in the matrix of cartilage are synthesized by cells embedded within the matrix. The matrix is actively synthesized during embryonic development of certain tissues and during periods of growth. The rates of synthesis and degradation of the matrix are less during adult life. However, throughout life, a continual slow synthesis and degradation of cartilage occurs, particularly in response to the pressures associated with physical activity.
Cartilage itself has several different functions in the body. During embryonic development, transient tracks of type II collagen and probably other components of cartilage are formed in many structures. The tracks appear to serve as a guide for cell migration and a template for formatting of skeleton and associated structures. In addition, cartilage serves as a precursor structure for many bones. During the development of long bones such as those of the arms and legs, cartilage is part of the growth plate in which cell growth occurs. More specifically, the cartilage grows away from the midpoint of the long bone and is continually degraded and gradually replaced by bone itself. An additional function of cartilage is to give shape and form to tissues such as the nose and ears. Many of the macromolecules found in cartilage are also present in the vitreous gel of the eye and account for the high viscosity of the vitreous. Still another major function of cartilage is to provide strength and resilience to structures such as the intervertebral disc of the spine. In joints, it provides not only strength and resilience, but also the smooth surfaces for motion under heavy loads.
The degeneration of joint cartilages that occurs in osteoarthritis is caused by a failure of the cartilage to maintain its structural integrity. In this process, the cartilage surface is eroded by physical pressures and is not adequately replaced by the new synthesis of cartilage. Instead of adequate repair of cartilage, secondary changes occur in the joint surface and in the joint. These changes include, for example, inflammatory responses characterized by invasion of white cells and macrophages, abnormal deposition of mineral in the form of calcium and phosphate within the joint space and in the cartilage itself, deposition of fibers of type I and other collagens that are not normally part of cartilage or the joint, abnormal growth of cartilage cells and matrix at locations adjacent to the joint surface and abnormal calcifications of the joints and associated structures. As part of the complex changes that occur, the cells of the cartilage or the invading cells from the blood stream begin to secrete degradative enzymes that further contribute to the degradation of the joint structures.
In the more severe diseases of cartilage known as chondrodysplasias, serious defects in the formation of cartilage are apparent early in life and there is a failure of joints to develop their normal size and shape. There is also a secondary failure of bone growth seen in these diseases. Moreover, there can be a failure of normal development of many tissues such as failure to achieve closure of normal partitions between oral and nasal passages, known as cleft palate, and improper development of the vitreous gel of the eye that causes severe myopia and retinal detachment.
Research has demonstrated that some forms of osteoarthritis and related conditions are caused by mutations in the genes that code for and, therefore, determine the structure of the collagens that are the major source of the strength of cartilage. Mutations of collagen that have been defined include, for example, mutations in the gene for type II collagen and its precursor type II procollagen. These mutations are of two general kinds. One kind of mutation decreases the synthesis of type II procollagen. The second kind of mutation leads to the synthesis of a defective form of type II procollagen. As a result of these mutations, there is either a decrease in the normal level of type II collagen in cartilage and other tissues that contain the protein or the formation of abnormal type II collagen fibrils that do not have the strength of normal type II collagen fibrils and, therefore, cause the cartilage of joints to be degraded by normal wear and tear. The two kinds of mutations can also produce drastic effects during normal growth and development. As a result, some individuals who inherit some of the mutated genes develop severe chondrodysplasias and die in utero or shortly after birth. Alternatively, such individuals can have serious deformities such as dwarfism which shows severe malformation of joints and may be associated with conditions of severe myopia, myopia with retinal detachment and blindness, cataracts, cleft palate, and unusual facial appearance. Other similar mutations in the same genes may produce much milder effects and cause progressive generalized osteoarthritis in which affected individuals are apparently normal until middle age when they develop progressive stiffness, pain and then immobility of many joints. Mutations of the gene for type II procollagen and collagen have been shown to cause these disorders. Research suggests that some of the conditions are caused by similar mutations in other genes that code for other structural macromolecules found in cartilage which contribute to its normal resistance to wear and tear. Methods and compounds are therefore desired for the analysis and detection of mutations in genes, both for type II procollagen and for a series of known collagens that are components of cartilage (types VI, IX, X and XI), as well as for still undiscovered collagens and other structural proteins that contribute to the normal strength and function of cartilage.